Water, or other similar fluids, is often used either for cleaning or cooling a machine, object, person, animal, or other entity. In the process of cleaning or cooling, water is often heated. The heated water is then rejected to the environment still warm. The energy contained in the waste water as it enters the environment can be considered heat pollution and so it is desired to remove this heat. Additionally, the heat in the waste water often has economic value, as it can be used to preheat another fluid and save energy on heating costs.
Recovering useful heat from waste water at a cost that is lower than the cost of the energy produced poses several challenges. Since water freezes at 0 C and boils at 100 C, there is a relatively small temperature range at which it exists at ambient pressure. One of the primary drivers of heat transfer is the temperature difference between the hot fluid, and the cold fluid to which the heat is being transferred. When the temperature difference is low, the heat transfer potential is limited. Additionally, the hot waste fluid typically is at ambient pressure and slow moving. Typically slow moving fluids at ambient pressure have low rates of heat transfer and also have increased risk of fouling heat transfer surfaces. These characteristics thus lead to the desire to enhance the heat transfer characteristics of the heat exchanger. However, due to the limited heat exchange potential, there is also limited benefit in adding enhancements to the heat exchanger which can increase the cost.
The prospect of recovering waste and turning it into useful energy has captured the imagination of many inventors. However, due to the challenges described previously, almost none of these technologies has ever been practically implemented. One notable exception is a technology which includes a vertical copper drain pipe, with a copper coil wrapped around the outside. This technology was first patented by Vasile et al and marketed under the name Gravity Film Heat Exchanger GFX. The inventors noted that in a vertical drain line, waste water tends to cling to the walls of a pipe in a falling film. It was additionally noted that water in a falling film has a high heat transfer coefficient, meaning that it gives off its heat relatively well and thus the heat exchanger was relatively efficient. Additionally, since it was made primarily of standard plumbing copper components, it was simple enough to be implemented and sold in the marketplace. Since this original invention, there have been several imitators and several improvements of this core technology. However, the technology did have very important limitations. Due to its reliance on a falling film of waste water, it could only be installed in vertical orientation. Further to this, several parties have raised doubts about the effectiveness of such technology in actual real life installations. There is very little data that exists about the performance of these devices in real life installations. There is doubt that the falling film is an ideal scenario, primarily achieved in laboratory settings. In real-life installations, it may be the case that other factors may impact such a falling film and that some portion of the waste water will drain through the center of vertical pipe, without making contact with the walls, and thus will not transfer any heat.
A need for a heat exchanger that could be mounted horizontally to overcome the limitations of these vertical heat exchangers was disclosed by Crump. Crump observed that drain pipes are typically oversized. Thus in normal operation, when a fluid is passing through a drain pipe, the drain pipe is not full. In the case of a horizontal drain pipe, the fluid tends to fill the bottom ⅓rd of the pipe. Crump thus disclosed several versions of a pipe in pipe heat exchanger designed to extract heat only from the bottom portion of a drain pipe. Specifically, Crump disclosed a double pass, and triple pass, channel beneath the drain pipe. He also disclosed a serpentine channel. Although there is no data provided by Crump on heat transfer efficiency of his heat exchangers, it is not likely that such a design would provide much heat transfer. The reason for this is that the cold water in the channel below the drain pipe is not mixed in any way. The cold water channel will have a large temperature gradient. The water that is at the top of the channel will heat up, but because higher temperature water rises, very limited heat will travel to the water in the lower portion of the channel. There is nothing in the channel to force the cold water at the center and bottom of the channel to reach the top and contact the warm portion of the channel wall which is in contact with the warm drain pipe. Additionally, the choice of a pipe in pipe design makes the heat exchanger prohibitively expensive. Pipes require a given thickness in order to retain their shape against pressure. As the pipe gets larger, thicker walls are required. The wall thickness required to retain pressure for a large tube at the outside of the drain pipe will be significant, and much more than if the second channel had been made from a series of smaller tubes than one large tube. Thus although Crump did make some useful observations, there are many limitations to the heat exchanger that Crump designed, and there is no evidence that it was ever installed in the field.
Cardone discloses a similar concept to Crump, except that instead of a tube in tube design, he discloses a top plate with a structure beneath the top plate that forces water into a similar serpentine pattern as disclosed by Crump. Although heat transfer data is not disclosed in this application either, it suffers from the same limitations as the Crump design. There is nothing in the heat transfer channels beneath the top plate that force water in the center and bottom of the channel to make contact with the warm top plate. Thus each channel of the serpentine heat exchanger will have a large temperature gradient with only a very small portion of the channel actually exposed to the warm top plate. Additionally, as in the Crump design, there also are limitations on the usefulness of the device due to its proposed construction. In one embodiment, Cardone discloses a large top plate that is a pressure containing wall. This requires a very thick and expensive plate as well as risky and difficult joining of the large top plate to the serpentine structure. Another embodiment disclosing tubes is disadvantageous due to the large number of welds required to make a serpentine structure using cut and welded tubes.
Studer additionally discusses a heat exchanger primarily designed to recapture waste heat from a horizontal drain pipe. However, the Studer patent application is concerned with placing a sheet of copper into a sewer pipe. The copper plate kills bacteria. In some regions, copper is not accepted as a sewer material and so the addition of copper sections near the heat transfer surfaces is necessary to kill bacteria that could foul the heat transfer surfaces. The present application is concerned with recapturing waste heat in a similar fashion except before reaching the sewer. In this case, where copper is required for its bacteria killing properties, it is typically accepted.
Disclosed herein is the addition of a turbulator to the cold fluid channels of the heat exchanger. Some of the functions of such a turbulator can be to heat the water at every level throughout the cold channel, not just on the top surface, and also to simplify the overall construction of the heat exchanger in order to reduce its cost.
In so far as a turbulator is a key component of this patent application, it is important to discuss prior art related to this. Maschio discloses a screw type insert into an oil filled electrical cable in order to locally increase the ability of the oil to absorb heat from the cable. The screw like insert into the round channel forces the oil to travel in a helical flow pattern. This increases the local heat transfer characteristics of oil and thus it is able to provide greater local cooling. However, this device disclosed by Machio is a simplistic device that only allows one helical flow path through cable. Although it is not disclosed how such a device is fabricated, it requires the complicated process of converting a strip of material into a helix. An improved turbulator specifically designed for a heat exchanger is thus required.
Based on these limitations in the prior art, we have developed an improved heat exchanger and turbulator combination. This combination results in a highly effective heat transfer performance as well as a cost effective design.